1. Field of the Invention
The present invention relates to a digital satellite communication system and, more particularly, to a satellite communication transmission control system which uses one central station and a plurality of small aperture terminals and performs frequency division multiple access communication via a satellite repeater, and a small aperture terminal.
2. Description of the Prior Art
There is known a satellite communication method for performing frequency division multiple access (to be referred to as FDMA hereinafter) communication of exchanging modulated carriers between one central station and a plurality of aperture terminals or between the plurality of aperture terminals via a satellite repeater in a digital satellite communication system constituted by the central station and plurality of aperture terminals.
In the FDMA satellite communication system, the precision of the center frequencies of modulated carriers must be set sufficiently high so as not to influence modulated carriers adjacent along the frequency axis of a satellite repeater. In other words, the central station and aperture terminals are always inhibited from transmitting unstable signals in transmitting modulated carriers.
When a plurality of modulated carriers A, B, and C are emitted toward a satellite repeater, as shown in FIG. 1A, no problem arises as far as the center frequencies of the modulated carriers have high precision. If, however, the center frequency of a given modulated carrier suffers a large error, this modulated carrier interferes with an adjacent modulated carrier and is adversely affected by itself, as shown in FIG. 1B.
In conventional radio communication, a known transmission control system in a transmitter is to prevent transmission of a transient unstable transmission signal upon turning on the transmitter.
For example, Japanese Examined Patent Publication No. 60-056778 discloses a radio transmitter which comprises a timer (power-on timer) for turning on only the power supply of a circuit on an antenna side with a delay in order to prevent any error caused by partial oscillation during feedback from the crystal oscillation circuit to the antenna upon turning on the power supply, and which transmits a transmission signal from the antenna after a crystal oscillation circuit is turned on. Japanese Patent No. 2944480 discloses a transmission output control system in which a transmitter comprises a controller (CPU), hard controller, and power-on reset circuit (power-on timer), a transmission output is controlled by the hard controller upon turning on the power supply, and after the controller (CPU) is activated, the transmission output is controlled in accordance with reception level information from a partner receiver.
Japanese Unexamined Patent Publication No. 64-001335 discloses a transmitter with a noise generation prevention circuit that detects noise generated from a digital phase comparator in the automatic phase controller (APC) of the transmitter, and turns on the output-side switch of the transmitter a predetermined time after stopping generation of noise, in order not to generate noise on the receiving side even if the voltage-controlled oscillator (VCO) of the APC having a reference frequency oscillator is activated from a frequency different from the center frequency based on the reference frequency oscillator when the transmitter is turned on. Japanese Unexamined Patent Publication No. 09-135178 discloses a transmission output control system which uses an alarm detection circuit for monitoring an output from a phase detector (phase comparator) in a similar APC and controlling ON/OFF switching of transmission, and a power-on reset circuit (power-on timer), and which inhibits ON-switching of transmission by the power-on reset circuit (power-on timer) upon turning on the power supply, thereby preventing fluctuations in transmission output caused by unstable operation of the alarm detection circuit.
As described above, each aperture terminal or the like executes communication by the FDMA method via a satellite repeater in the digital satellite communication system for performing communication between one central station and a plurality of aperture terminals or between a plurality of aperture terminals via a satellite repeater. Transmission signals must be transmitted while their frequency bands are prevented from overlapping each other. Particularly, transmission of an unstable transmission signal must be prevented upon turning on the power supply.
For example, the frequency band of the Ku band often used in a satellite communication system in which the aperture terminal is formed from a very small aperture terminal ranges from 14.0 GHz to 14.5 GHz in uplink to a satellite. The transmission frequency error of the modulated carrier is 1 ppm (=1×10−6). This error corresponds to 14 kHz, which cannot be ignored in a modulated carrier having a modulation rate of 32 kbps in widely spread ADPCM voice communication digital satellite communication. The tolerance transmission frequency error as the error of the transmission frequency allowed in this communication system is about ±0.1 ppm or less.
The precision of the transmission frequency of a modulated wave emitted by an aperture terminal depends on a transmission frequency converter (to be referred as a U/C (Up Converter) hereinafter) in the transmitting section. The U/C has a local oscillator, which determines the center transmission frequency of the modulated carrier.
The local oscillator is formed from a synthesizer type PLL circuit. A crystal oscillator having a frequency of several ten MHz (e.g., 10 MHz) is used as an oscillation source. This frequency is multiplied and used by the local oscillator (e.g., for a 10-MHz oscillation source, a multiple of 1,400 yields 14 GHz). If the oscillation source has an error of 1 ppm, this error is also multiplied and appears as an error of 1 ppm even in the local oscillator.
The precision of the oscillation source determines the center transmission frequency of the modulated carrier. For this reason, the crystal oscillator serving as an oscillation source must maintain high precision.
In general, a temperature-compensated crystal oscillator called OCXO (Oven Controlled Crystal Oscillator) (to be also referred to as an OCXO hereinafter) is widely used as a crystal oscillator which can maintain high precision. The OCXO is a thermostatic crystal oscillator which incorporates a heater for generating heat, has a crystal oscillator or crystal oscillation circuit confined in a stable-temperature oven, and realizes very high frequency stability. In general, the OCXO precision can be kept at about ±0.005 ppm to ±0.01 ppm.
When the OCXO is activated (circuit is powered on), the frequency precision is as low as several ten ppm until the internal oven is warmed up and serves as a thermostat to obtain a stable temperature. The startup time until the frequency is stabilized is several min under the present circumstances.
In other words, during several min until the frequency is stabilized after power-on operation, the OCXO cannot be used as the oscillation source of the PLL constituting the local oscillator. If a transmission signal is transmitted during this period, interference with an adjacent carrier occurs.
FIG. 2 is a graph showing an example of the oscillation frequency error of the OCXO upon power-on operation. The abscissa represents the lapse time after the power supply is turned on, and the ordinate represents the error from a rated frequency. For example, if a modulated carrier is emitted in the presence of an error of 10 ppm, the center frequency of the modulated carrier shifts to a frequency different by 140 kHz in the 14-GHz band, and the carrier interferes with an adjacent carrier.
The transmission control system disclosed in the above reference has been known as a conventional technique for preventing transmission of an invalid signal upon turning on the power supply. The radio transmitter disclosed in Japanese Examined Patent Publication No. 60-059778 uses a power-on timer for inhibiting transmission only during a predetermined time simply after the power supply is turned on. This control system starts transmission after a predetermined time even if the oscillator cannot reach a steady state owing to any fault and the frequency error from an original frequency is large. This system cannot be applied to FDMA transmission control. The transmission output control system disclosed in Japanese Patent No. 2944480 uses information received from a partner receiver. However, the received information is used for only the reception level so as to control the transmission output level. After the power-on timer operates, a transmission signal is output regardless of a frequency error from the center frequency of a transmission carrier. For example, even if an oscillator for determining a transmission frequency malfunctions and oscillates at an abnormal frequency, a transmission signal is output at the abnormal frequency after a predetermined time. Also, this control system cannot be applied to FDMA transmission control.
The transmission control systems disclosed in Japanese Unexamined Patent Publication Nos. 64-001335 and 09-135178 monitor an output (APC voltage) from a phase comparator. Even when the frequency of a reference frequency oscillator suffers a large error of, e.g., about several ten ppm from a specified frequency, the frequency is determined normal. This control system undesirably transmits a carrier having a large frequency error.
From this, the prior arts cannot be applied to transmission control of the modulator of a central station when FDMA communication is performed via a satellite repeater between one central station and a plurality of aperture terminals or between the plurality of aperture terminals in a digital satellite communication system constructed by the central station, the plurality of very small aperture terminals, and one communication satellite.
The influence of the frequency error of a transmission signal from each aperture terminal can be avoided by setting a wide occupied bandwidth usable in the satellite repeater. This is disadvantageous in effective use of the frequency and the cost.